1. Field of the Invention
The present invention relates to a conical-mode helical antenna for use in a mobile station of a mobile satellite communication system, for example.
2. Description of the Related Art
In a mobile satellite communication system, uniform antennas in the azimuth direction have been used in mobile stations, since the uniform antennas in the azimuth direction do not need a tracking system for a satellite. Particularly, conical-mode helical antennas have directivity in the elevation direction, and therefore, the radiation directivity of these antennas can be toward the satellite, so that the gain can be increased.
A prior art conical-beam helical antenna has a coaxial cable and a helical conductor wound on the coaxial cable. In this case, the turns of the helical conductor are uniform along the coaxial cable, i.e., a spacing between the turns is definite along the coaxial cable. This will be explained later in detail.
In the above-mentioned prior art conical-mode helical antenna, however, since a radiation pattern is determined unambiguously by a spacing between turns, a diameter of the turns and the like, the radiation pattern is very narrow. In addition, the direction of the maximum beam of the radiation pattern is dependent upon the frequency of radio waves, and therefore, the gain at a particular elevation angle such as a satellite angle fluctuates. Thus, it is impossible to cover a broad elevation angle range.